BOARD02642

• Board Meeting Date: 1/12/1998
• Description: Flood Section - Floodplain Designations
• Board Meetings - Doc Type: Memo

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represented by the previously approved and designated CWCB Floodplain Resolutions for the
100-year Floodplain. These changes in the floodplain conditions and hydrologiclhydraulic
characteristics were evident for CWCB Floodplain Resolution 97-456.

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As a result of these major changes in the engineered/mapped I OO-year floodplain, it was the
Board's staff opinion that a technical review be performed for all previously designated 100-year
floodplains regarding the Pawnee Creek Flood Overflows into the City of Sterling. The 1997
flood event was an actual flood event which nearly approximated a I OO-year discharge at a
common design location. The staff's engineering investigation resulted in the following findings
and issues regarding Pawnee Creek Flood Overflows.

Key Issues:

. The hydrology values at the common design point (Riverside Cemetery) are:
the 1965 and FEMA design 100- year discharge is 5,000 cfs;
the NRCS 1992 100- year discharge is 4,242 cfs; and
the August 1997 CWCB and Corps of Engineers 1997 flood event discharge is 3,800 cfs.

. The hydrology values set forth in Floodplain Resolution 97-456 at the site of interest are:
the design discharge for the 100- year floodplain information study is 600 cfs; and
the CWCB and Corps of Engineers estimated 1997 flood event discharge is 2,800 cfs.

. The hydraulic determinations set forth in Floodplain Resolution 97-456 at the site of interest
are:
the flood depths in the Icon floodplain study dated February 3, 1997 are in the general
range of
0.3 foot; and
the flood depths during the July 30-3 I , 1997 event were in a general range of 1.3 to 1.8
feet as surveyed by the CWCB and Corps of Engineers staffon September 11, 1997.

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In summary, the actual flooding conditions that occurred during the July 30-3 I, 1997 flood event
were significantly different from the floodplain delineations shown on the existing FEMA or
ICON floodplain mapping for Sterling, Colorado. There are several physical and hydrological
reasons why the actual 1997 flood conditions differed from the engineered/mapped 100-year
floodplain.

COMMUNITY RESPONSE:

On September 10, 1997, the CWCB staff discussed its concerns regarding Floodplain Resolution
97-456 with representatives from the City of Sterling. It was the City's request that the CWCB
proceed with its action under Rule 9.0.2 (Board Initiative) regarding the rescission of Floodplain
Resolution 97-456. The Board gave its 45-day notice for a Recommendation to Rescind
Floodplain Resolution 97-456 by letter dated October 6,1997. See attached.

RECOMMENDATION

The floodplain information for Resolution 97-456 which was approved at the Board's May 12,
1997 meeting under the Board's "Floodplain Designation and Approval Rules" be rescinded by
the Board.

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